A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
A bibliography of references cited herein is included at the end of this specification. The references listed in the bibliography, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to manufacturing methods and systems (collectively referred to as "processes") for the freeform shaping of calcium-containing powders. This invention more specifically relates to processes for shaping bone implants from various calcium phosphate powders and polymer-emulsion binders. Certain embodiments of these processes focus on the use of a Selective Laser Sintering.TM. ("SLS.TM.") device to automatically and selectively fuse the polymer binder. In such processes, complex three-dimensional objects may be built by selectively fusing successive thin layers of the powdered material.
2. Description of the Related Art
Much attention has been given in the art to the development of materials to assist in the regeneration of bone defects and injuries. In 1926, DeJong observed the similarities between the powder X-ray diffraction pattern of the in vivo mineral and the hydroxyapatite (Ca.sub.5 B(OH)(PO.sub.4).sub.3, "CHA"). Calcium compounds, including calcium sulfate (Nielson, 1944), calcium hydroxide (Peltier, 1957), and tricalcium phosphate ("TCP") (Albee et al., 1920), have been observed to stimulate new bone growth when implanted or injected into bone cavities (Hulbert et al., 1983). These materials also exhibit good biocompatibility and compositional similarities to human bone and tooth and can serve as resorbable or non-resorbable implants depending on their degree of microporosity.
Some TCP implants are known to be readily resorbable. For example, sintered TCP plugs with pore sizes between 100-200 microns have been implanted in rats (Bhashar et al., 1971). Very rapid bone formation was reportedly observed at three days after implantation, and highly cellular tissue, consisting of osteoblastic and fibroblastic proliferation, was found within the pores. At one week, the size of the implant was reduced, and new bone formation was extensive. After two weeks, connective tissue had infiltrated throughout the ceramic. During the next four weeks, the boney material within the ceramic continued to mature. Electron micrographs indicated that within clastlike cells, ceramic could be depicted in membrane-bound vesicles. The authors concluded that TCP implants were biodegradable, via phagocytosis, the ceramic did not elicit a marked inflammatory response, and connective tissue grew rapidly within the pores.
Similar results have also been reported by Cutright et al. (1972) who also implanted TCP in rat tibiae. In this study, the ceramic cavities were filled with osteoid and bone after 21 days and the TCP implant was no longer detectable after 48 days.
Larger implants in dogs are reported to elicit slower responses. Cameron et al. (1977) found that TCP implants in dog femurs were completely infiltrated with new bone by four weeks. However, after six weeks, the rate of new bone growth had slowed as the TCP was resorbed. Additionally, only 15% of a 2 cm.times.2 cm iliac TCP implant in dogs was resorbed after 18 months (Ferraro et al., 1979).
Koster et al. (1976) reported the testing of the calcium phosphate formulations monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, tetracalcium phosphate, and combinations consisting of 20% monocalcium phosphate and 80% of either di-, tri- or tetracalcium phosphate as implant materials in dog tibiae. These investigators tested both dense ceramics and porous ceramics with pore sizes between 800-1000 microns. They reported that tissue compatibility is dependent on the CaO/P.sub.2 O.sub.5 ratio. All materials with ratios between 2/1 and 4/1 are compatible with the optimum ratio being about 3/1 for TOP. After 10 months, Koster et al. (1977) found that tetracalcium phosphate was resorbed only to a minor extent, but that TCP demonstrated lamellar bone growth throughout its pores. Both were found to be tissue compatible. The authors stated that the 3/1 material was not as strong as the 4/1 material and suggested that TCP should be used only in low stress areas while tetracalcium phosphate could be used in high stress environments.
Jarcho et al. (1976, 1977) reported the development of a process for preparing dense, polycrystalline, calcium hydroxyapatite (CHA), with the empirical formula 2(Ca.sub.5 (PO.sub.4).sub.3 OH) or (3Ca.sub.3 (PO.sub.4).sub.2)Ca(OH).sub.2. In this study, plugs were fabricated at 100% density and implanted in dogs. No evidence of tissue inflammation occurred, and in contrast to the porous TCP implants described above, little resorption or biodegradation was observed after six months.
Holmes (1979) reported that resorption did occur in porous CHA structures. These results led deGroot (1980) to suggest that all calcium phosphates are degradable (resorbable), but the rate is determined by the degree of microporosity. A dense calcium phosphate with negligible porosity would thus degrade only nominally. These results seem to be verified by Farris et al. (U.S. Pat. No. 4,673,355), who claim biocompatible materials with good properties over the range of Ca/P atomic, or molar, ratios from 0.1 to 1.34. (All patents and patent applications cited herein are incorporated by reference.) These ratios convert to CaO/P.sub.2 O.sub.5 ratios between 0.2 and 2.68, lower than the 3.0 ratio suggested above. They suggest that the Ca/P or CaO/P.sub.2 O.sub.5 ratio is not critical for implant applications. Ca/P ratios in the range 0.1 to 2.0 probably show satisfactory biocompatibility. Capano (1987) found that a Ca/P ratio of 0.5, which corresponds to calcium metaphosphate ("CMP"), has the best biocompatibility when implanted in small animals.
As the apatites are nearly identical in properties and chemical compositions to bone and tooth enamel, a considerable amount of synthetic effort has been done in this area. Patents in this area include: U.S. Pat. Nos. 4,046,858; 4,274,879; 4,330,514; 4,324,772; 4,048,300; 4,097,935; 4,207,306; and U.S. Pat. No. 3,379,541.
Several patents describe methods for treating apatite materials to render implantable shapes. These methods of heating and compaction under pressure in molds produce solid porous articles in various shapes. These patents include: U.S. Pat. Nos. 4,673,355; 4,308,064; 4,113,500; 4,222,128; 4,135,935; 4,149,893; and U.S. Pat. No. 3,913,229.
Several patents speak to the use of laser radiation to bond apatite materials to tooth and other surfaces, for example, U.S. Pat. No. 4,673,355 and U.S. Pat. No. 4,224,072.
Other patents describe the use of particulate or compacted apatite in conjunction with various compounds, filler, and cements, for example, U.S. Pat. Nos. 4,673,355; 4,230,455; 4,223,412; and U.S. Pat. No. 4,131,597.
The above discussion indicates that calcium phosphates or compounds, such as CHA that are substantially TCP (Monsanto, for example, markets CHA as TCP), are useful for a variety of bioceramic applications because they are biocompatible and can be fabricated into shapes that have a desirable combination of strength, porosity, and longevity for particular sorbable and non-sorbable needs.
Virtually any calcium and phosphate source can be used to prepare materials of interest. An important issue is the ratio of Ca to P or, as it is usually expressed, CaO to P.sub.2 O.sub.5, molar ratio in the reactant mixture. For example, one can prepare monocalcium orthophosphate monohydrate from the reaction of CaO with orthophosphoric acid, H.sub.3 PO.sub.4, as shown in equation 1: ##STR1##
One could also react CHA with H.sub.3 PO.sub.4 to achieve the same product, as shown in equation 2: ##STR2##
Heating the orthophosphate hydrate can lead to a variety of known products, depending on the firing temperature used, as shown in equations 3-8: ##STR3##
The .alpha.-, .beta.-, and .delta.-forms of calcium metaphosphate are different crystal structures of the same chemical compound that happen to be stable at different temperatures. Tricalcium phosphates can be easily obtained from CHA by simply lowering the Ca/P ratio, as shown in equation 9: ##STR4##
According to Mcintosh et al. (1956), the orthophosphate hydrate can be converted to two crystalline forms by heating, as shown in equations 10-11: ##STR5##
Similar reaction schemes can be written for producing di-calcium and tetra-calcium phosphates from CHA or any other calcium source by reacting with orthophosphoric acid or any other P.sub.2 O.sub.5 source. The chemical and crystalline forms of the final product are simply set by the Ca/P or CaO/P.sub.2 O.sub.5 molar ratio and the final temperature.
Five calcium phosphates which exhibit different x-ray diffraction patterns are known to be precipitated from aqueous solution at normal pressure (Van Wazer, 1958). These are Ca(H.sub.2 PO.sub.4).sub.2, Ca(H.sub.2 PO.sub.4).sub.2.H.sub.2 O, CaHPO.sub.4, CaHPO.sub.4.2H.sub.2 O, and crystalline precipitate of variable composition of hydroxyapatite with the base formula Ca.sub.5 (OH)(PO.sub.4).sub.3. Various forms of calcium phosphate compounds, Ca/P ratio range from 0.5 to 1, are prepared from the reaction of calcium hydroxyapatite with phosphoric acid.
Thermally dehydrated calcium phosphates are known to form a CaO and P.sub.2 O.sub.5 binary system. For the CaO and P.sub.2 O.sub.5 binary system, the chain phosphates appear between the orthophosphate (mole ratio of CaO/P.sub.2 O.sub.5 of about 3) and metaphosphate (mole ratio of CaO/P.sub.2 O.sub.5 of about 1) or ultraphosphate (mole ratio of CaO/P.sub.2 O.sub.5 of less than 1). The metaphosphates, in particular, generally exhibit very high degrees of polymerization and good mechanical properties. In this binary system, with a mole ratio of CaO/P.sub.2 O.sub.5 less than 55/45, a glass-like structure forms from the melt which has mechanical properties similar to those of natural teeth (Yoshihiro, 1975).
Many studies and methods, from powder compaction sintering to hot isostatic pressing, have been reported for the fabrication of CHA implants. However, sintered CHA materials by conventional techniques are generally as weak as sea coral even at high compacting pressure, because CHA decomposes at temperatures lower than the required temperature for sintering.
Some more recent advances are the development of hydroxyapatite (CHA) and calcium phosphate powders that can be processed to yield porous resorbable bone facsimiles (U.S. Pat. No. 4,673,355); the development of the SLS.TM. process for directly shaping complex porous structures from thermally fusible polymer/ceramic powders without molds (U.S. Pat. No. 5,076,869); the development of low temperature infiltration and cementing techniques to prepare and replace the polymer binder with ceramic binder (U.S. Pat. No. 5,284,695); and the development of techniques for converting computed tomographic ("CT") information into three-dimensional mathematical files that can automatically guide the SLS.TM. process (Levy et al., 1992; Levy et al., 1994).
More recent work has been directed at expanding the utility of the SLS.TM. apparatus by preparing polymer-coated ceramic powders from spray dried mixtures of water, inorganic particulate, and a custom-synthesized, emulsified, nanometer-sized, polymer binder (Barlow, 1992; Vail et al., 1992). Ceramic composites made by this approach are relatively large, 10-50 microns, agglomerates of polymer-coated inorganic particles. These agglomerate powders may spread easily into uniform layers and fuse readily in the SLS.TM. machine to yield porous "green" parts that have relative densities near 50%, excellent connected internal porosity, and sufficient strengths to be easily handled and shipped. Interconnected pores in bioceramics are often difficult to achieve and are very important in fostering bone growth and for preparing metal matrix/ceramic parts, artificial hips.
Polymethyl methacrylate (PMMA) has also been used to form green composites with alumina and with silica/zircon (U.S. Pat. No. 5,284,695). In this process, an appropriate ceramic silicate colloid is used to infiltrate the connected pores of the polymer-bound green part, the colloid is solidified below the fusion temperature of the binder to maintain part geometry, the binder is then thermally removed and the part fired at typically 1000.degree. C. to form porous, all ceramic parts that are suitable for use as cores and molds for metal castings. Such parts typically have only a 1% linear shrinkage, relative to the green state. Their strengths and porosities can be adjusted by additional infiltration and firing treatments.
Lagow and co-workers have recently described the chemical synthesis of high strength CHA (U.S. Pat. No. 4,673,355) and long-chain calcium polyphosphate bioceramic powders ("CPB") (Capano, 1987; Nelson et al., 1993). CPB powder is a pure calcium phosphate material with condensed phosphate chains (as shown below) with degrees of polymerization often greater than 120. ##STR6##
These materials produce sintered materials that have compressive strengths greater than 200,000 psi and flexural strengths in excess of 20,000 psi. These strengths are about twice that of porcelain used to make dental crowns. Using the Lagow CHA material, Lagow and Friedman have recently completed the first successful, year duration, mandible implant in a canine. Work with CPB implants has demonstrated by electron microscopy backscattering that new bone growth occupied nearly 55% of the volume of a CPB implant in the alveolar (tooth bearing) ridge of a dog, after only four months (Nelson et al., 1993). This rate of resorption and replacement by living bone in CPB is about twice as fast as that in CHA.
The lack of suitable bone replacement is a general problem that can be potentially solved by the development of synthetic bones and bone templates that are converted to bone by the body. Bone banks currently provide gamma radiation-treated cadaver bones for various orthopedic and reconstructive purposes in a world-wide business. Appropriate geometries are not always available from these sources, and there is some concern about the transmission of HIV and other diseases. For example, in connection with spinal fusions, there is a substantial need for wedge materials that can provide support and promote the deposition of additional bone. These needs could be rapidly multiplied, provided viable materials and processes could be developed to readily provide bone materials that are shaped to the needs of each individual patient.
Facial and cranial reconstructive surgery is an area where the need for individual implant geometries is especially critical. At present, such reconstructions tend to be very difficult surgical procedures, typically involving highly skilled grafting with allogenic bone. The method and system of the present invention can be utilized to accurately construct a complete facsimile bone structure, suitable for implantation, employing geometric information that is obtained from either CT data or a Computer Aided Design ("CAD") software package.